There are a number of vectors of food-born illness, but few are as dangerous as the salmonella bacterium.
The vast majority of food-born illnesses are due to toxicity from the bacterial decomposition of meat. All animals have bacteria living on their skin. When someone dies, those bacteria begin feeding on the corpse and like all living things, when they eat, they poop. In the case of almost all of these meat-eating bacteria, the substance they excrete is poisonous.
In the case of beef, the rate of bacterial spreading is relatively slow, and the toxicity of the bacteria involved is relatively mild. So long as proper time-temp control is observed, the meat shouldn’t have ever developed enough toxin to hurt you.
Salmonella on the other hand, is a bacterial infection. If there is any of the bacteria alive in the chicken, it has a serious chance of beginning to reproduce within your body. And while this is seldom fatal among healthy adults, it is never pleasant. Further, someone with a weakened immune system, like an elderly person or a child, might actually die from the infection.
So food preperation laws in the US require all chicken to be cooked to at least 165 degrees before being served, in order to kill off as much of the bacteria as possible.